Traveling wave electron discharge devices



March 4, 1958 H. P. ISKENDERIAN TRAVELING WAVE ELECTRON DISCHARGE DEVICES Filed Jan. 29, 1953 2 Sheets-Sheet l N .4 q l mm a R m fl I l I l llWrN I I l l ll Q E mw 4 1 l l l l l 1 n l l W T .1 m A l l l 1 l 1 4 fl1 1 M MY +++H l l l l l I l1 AB v AXES H Q \U zbw Q m March 4, 1958 H. P. ISKENDERIAN TRAVELING WAVE ELECTRON DISCHARGE DEVICES 2 Sheets-Sheet 2 Filed Jan. 29, 1953 INVENTOR HA/fi R ISKENDE/P/AN BY AZ);

ATTORN EY United States Patent Ofifice 2,825,840 Patented Mar. 4, 1958 TRAVELING WAVE ELECTRON DISCHARGE DEVICES Haig P. Iskenderian, Elmhurst, Ill., assignor to International Telephone and Telegraph Corporation, a corporation of Maryland Application January 29, 1953, Serial No. 333,861

Claims. (Cl. 3153.5)

This invention relates to electron-optical systems for traveling wave electron discharge devices and more particularly to hollow cylindrical permanent magnet arrangements for such devices.

. The traveling wave type of tube is particularly useful in wideband microwave systems since it is capable of amplifying radio frequency energy over an unusually wideband of frequencies. The tube includes a form of transmission line, usually a helix, for transmission of microwave energy for interaction with an electron beam closely associated with the line. The helical characterishe of the transmission line is such that the axial velocityof microwave signals conducted along the helical path is approximately the same as or slightly slower than the velocity of the electrons of the beam whereby the electrical field of the microwave signals interact with the electron beam for amplification of the microwave signals.

In the passage of the electron beam from an electron gun along the length of the transmission line, there is a tendency for the electrons of the beam to deviate from a desired electron beam path and as a result causes an increased electron beam cross-section which eventually w ll become greater than the diameter of the helical trans mission line. This deviation of the beam is caused by the natural space-charge repulsion of electrons along the path of the helix, as well as by some types of electron guns which naturally project a convergent electron beam. In becoming larger than the diameter of the helical transmission line, the electron beam will transfer electron beam current to the helical line which is a feature undesired for maximum amplification, as well as requiring power dissipation on the helical structure. The use of magnetic fields to maintain electron beams uniform in diameter is substantially well understood, which is to say that the required magnetic field for any given application can be predicted. Heretofore, this problem of producing this magnetic field has been solved by employing electromagnets comprising a number of solenoid coils, ranging from one to four separate coils, coaxial of the electron beam path and extending substantially the entire length of the traveling wave propagating structure. These solenoids with their associated current supplying devices are arranged to produce a substantially uniform magnetic field whose lines of flux extend axially of the helical conductor for substantially its entire length to insure a pencil-like beam throughout the entire length of the transmission line.

Thus, traveling wave electron discharge devices have heretofore employed electromagnets to confine the electrons, produced therein by an electron gun, in an axial beam having a small, uniform cross-section throughout the length of the interaction or radio frequency section of a traveling wave device. In a number of traveling wave device applications these electromagnets with the number of coils employed and the circuitry required to establish the regulated current for flow therethrough to produce a desired magnetic field become cumbersome. Further, even with all the current regulating circuitry, the magnetic field produced by these electromagnets often varies enough to produce an undesired efiect of beam expansion, thus yielding undesired power operating characteristics. An ordinary permanent magnet of the bar or horseshoe type having associated therewith necessary pole pieces, may be employed to replace the electromagnets as disclosed in the copending application of J. H. Bryant and H. W. Cole, Serial No. 321,342, filed November 19, 1952, entitled Traveling Wave Electron Discharge Devices now abandoned. The employment of such permanent magnetic electron-optical systems provides a more economical beam focusing means than the electromagnet type electron-optical systems, however, the magnetic field thus provided is axial over a small portion only of the total length of the interaction section of a traveling wave device, the field having a tendency to decrease in magnetic strength in the central portion thereof. Therefore, it is the object of this invention to provide an electron-optical system of the permanent magnet type for obtaining a substantially uniform axial magnetic field distribution over at least of the length of the interaction section of traveling wave electron discharge devices.

Another object of this invention is the development of a law of magnetization applicable to a hollow cylindrical radially magnetized permanent magnet which yields substantially uniform axial magnetic field distribution over approximately 85% of the length of the cylindrical magnet.

A feature of this invention is a method of providing a hollow magnetized cylindrical shell suitable as the electron-optical system for traveling wave type of electron discharge devices. Two identical half cylinders, prior to fabrication into a whole hollow cylinder, are first radially magnetized separately to saturation. The magnetization being such that the inner wall of each half has the same polarity. The halves are then partially demagnetized in predetermined steps along the length to produce the desired axial magnetic field distribution according to the developed law of magnetization for hollow cylindrical permanent magnets.

Another feature of this invention is a method of providing a hollow magnetized cylindrical shell by first magnetizing a plurality of pie-shaped elements with the apex of each of the same polarity and then fabricate them into a hollow cylindrical magnet.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional view illustrating an application of an embodiment of this invention;

Fig. 2 in a longitudinal sectional view of the electronoptical system of Fig. 1 to aid in the development of the law of magnetization in accordance with the principles of this invention;

Figs. 3 and 4 are graphical representations of certain functions, M(x) and K" (xx and the magnetic field distribution of a cylindrical shell permanent magnet magnetized radially, respectively, aiding in the development of the law of magnetization;

Fig. 5 is a longitudinal sectional view illustrating the equipment employed in a method of radially magnetizing.

a hollow cylindrical shell; and

Fig. 6 is a view in perspective of another embodiment in accordance with the principles of this invention.

Referring to Fig. 1, a traveling wave type electron discharge device 1 is shown to comprise a cylindrical housing 2 of non-magnetic material containing in one end thereof an electron gun unit 3 and in the opposite end thereof a radio frequency interaction section 4. Thehousing 2. has mounted thereon and concentric thereto anv embodiment of an electron-optical system in accordance with the principles of this invention comprising- Patent No. 2,788,465. Disclosed therein is the structural.

arrangement of a traveling wave tube with the terminals for the electrodes of the gun unit at one end of the tube and the terminals for the radio frequency input and output connections at the other end of the tube. The input and output R.-F. connections disclosed therein are of the coaxial type, but may be adapted to function with other forms of waveguide coupling lines if desired.

Referring more particularly to interaction section 4, a radio frequency transmissionline 6 preferably in the form of a helix may be supported in a ceramic tubing positioned axially by the magnetic barrier members 8 and 9 and matching transformer sections 10 and 11 located respectively at each end of section 4. Itis preferred, however, to support helix 6 in 'a plurality of dielectric support rods 12 equally spaced about the helix. The transmission line 6 is preferably coated for substantially the first half of its length with lossy material such. as aquadag or the like to minimize the reverse R.-F. waves along the conductor 6 and the electric field thereof. If desired the propagating structure may assume other configurations such as a plurality of annular discs or plates, whereby the axial velocity of the radio frequency energy is made preferably slightly slower than the. velocity of the beamof electrons projected from u'nit3.

The magnet of the electron-optical system of this in.- vention comprising a' cylindrical shell 13 must be mag netized according to some prescribed law of magnetization to yield a substanially uniform field herein and for a major portion of the axis of said shell. The following analytical discussion in. connection with Figs. 2, 3, and 4 specifies one of the laws of magnetization for shell 13 which yields thet desired uniform axial field within the electron-optical system located concentric to the traveling wave tube 1 of Fig. 1.

First consider a cylinder shell 13 which has a distribution of radial magnetization M(x) as illustrated in Fig. 2 where M (x)=2l times surface pole strength m(x). The potential AV at an arbitrary point P or 14 on the axis of cylinder 13 a. distance: x -x from a circular element dx is given by:

3 AV [a 1 where a is equal to the average radius of cylinder 13. The. corresponding axial component of magnetic field AH at point 14 will be:

Equations 1' and 2 may be obtained byutilizing" the formulas and discussion relative thereto as found in the textbook, A. S. Ramsey, Electricity and Magnetism, second edition, Cambridge at the University Press, Section 8.71, page 201. The total axial magnetic field due Further utilization. of such a permanent magnet as herein disclosed it is desirous to obtain a uniform magnetic field H inside and on the axis of the cylinder shell. This implies that,

for a second approximation, with higher order approximations, if desired, being provided by higher order differentiations of- Equation 5 being set equal to zero.

It follows from Equations 6 and 7 that the magnetization function M(x) should be orthogonal to K and K" over the limits of integration of L to L. The determination of M(x) may beobtained by adapting a graphical method of analysis to satisfy Equations 6 and 7.

By setting Z=x-x in Equation 4 and differentiating thereis obtained:

It will be seen from Equation 8'that K (Z) is a symmetrical function, hence Equation 6 will be satisfied for all, M(x) which are anti-symmetrical, the simplest one being'M (x) =M x.

I To obtain a higher order of approximations for the desirableform offM(x)', representative of a more uniform magnetic field distribution, an attempt should be made to satisfy Equationl7'. A plot of K" (Z) is substantially illustrated in Fig. 3 by curve 15.

It will be understood from curve 15' of Fig. 3 that K (Z) is antisymmetrical; hence, M(x) which must also be anti-symmetrical to satisfy Equation 6, should satisfy Equation 7 over the limits of integration 0 to L, or L to 0.

In view of the apparent difiiculties in the integration of the form resents' the M(x) function as established by- Equations 10 wherein the portion 17 has "a slope'M and portions 18 have'a slope Mg.-

l u)'l"' From Equation 11 H(x is found to be a function of M M K, and other physical constants. in order to evaluate H(x it is necessary to determine the values of k and M M To do so the following boundary conditions may be selected as:

and

Manipulation of these two equations will determine the values of k and M /M substantially as follows. The values of the magnetic field H(x for x =0, L/ 2, 3L/4, and L, may be obtained from Equation 11. For the present case under consideration L/a is chosen to equal approximately nine.

If l-I()gH(L/2)=H(3L/4), it is possible to obtain from the above equations the values k=% and M =1.-3M Substitution of these values in the Equations 14 or in Equation 11 yields the following values for the magnetic field distribution at certain points along the axis of cylinder 13 and as substantially indicated by curve 19ofFig.4. Y

cylinder 13 as illustrated by curve 20 of Fig. 4. Therefore, the curve 20 represents the function 1 =Mi for e==--LtoL while the curve-19 represents the summation of the functions To obtain the magnetic field distribution as defined by Equation 11, it is necessary to establish a radial magnetization M(x) on cylinder shell 13 as represented by curve 16 of Fig. 3. Referring to Fig. 5, apparatus is illustrated which is employed in a method for realization of a radial magnetization M(x) to substantially satisfy Equation 11. The apparatus comprises a cylinder 21 having an inwardly extending flange 21a thereon substantially as shown to provide a substantially equal distribution of flux, a circular disc 22 closing one end of cylinder 21, and a solid cylindrical rod 23 axial with respect to cylinder 21 and movable longitudinally therein, said cylinder 21, disc 22, and rod 23 being composed of magnetic material such as soft iron. Associated with rod 23 and concentric thereto is a winding 24 having connected thereto a D.-C. current source 25 through a switch 26 which provides a reversing feature whereby the current through winding 24 may be reversed as desired.

The action of D.-C. current flowing through winding 24 is to establish a magnetic flux in rod 23 such that when shell 28 to be radially magnetized is inserted in the apparatus concentric to rod 23, the flux lines 27 will pass through shell 28 substantially at right angles to the walls of said shell. Thus, a path of magnetic material is provided for flux lines 27 through rod 23, shell 28 to be magnetized, cylinder 21, and disc 22 all of which provide together a means of radially magnetizing the shell 28, in sectional steps, as the shell is pulled out of the apparatus in steps about equal to length b shown on Fig. 5 after completion of each magnetization cycle. The length b" is preferably approximately equal to the square of the inner diameter of permanent magnet 28 divided by four times the average diameter of permanent magnet 28.

A method employed to provide a radially magnetized cylindrical permanent magnet to satisfy the analytical discussion herein expounded may be performed by the apparatus of Fig. 5 in'the following manner. The magnet 5' of Fig. 1 preferably consists of a cylindrical shell 13, Fig.2,- comprising two identical shells 28 and 29 whose inner diameters are sufficient to encircle tube 1. Each of the shells 28- and 29 are placed separately in the area between rod-23 and cylinder 21 to encircle rod 23 as indicated for shell 28 in Fig. 5. When placed in position the half shells may bemagnetized separately to saturation as described above, but, with reverse magnetic polarity. This reversed magnetic polarity is obtained by reversing the polarity of the magnetizing current from source 25 through means of reversing switch 26. After being magnetized separately to saturation these two half shells are then placed within the apparatus of Fig. 5 whereby a stabilizing operation of partial demagnetization takes place to obtain the magnetic distribution necessary to satisfy the law of magnetization M(x) for a particular application of this electron-optical system. This stabilizing operation is accomplished by demagnetizing the half shells 28 and 29 in steps along the lengths as prescribed by the Equations 10 by the action of properly controlling the demagnetizing currents in winding 24 for action on various portions of said shells, as the shells 28 and 29 are withdrawn a length 11 therefrom in predetermined steps to the left from disc 22, Fig. 5. It may be further required to withdraw rod 23 in a manner to aid the stepped withdrawal of the shells to obtain the desired law of magnetization.

It is recognized that the methods herein disclosed to radially magnetize a cylinder in achievement of a desired law ofmagnetization is not cut and dry, there -is definitely a trial and error situation present. However, proper manipulati-on of the demagnetizing current in conjunction With the stepped withdrawal of the rod 23, changing the length of the flux path through air and thereby the strength of said flux, will enable the realization of the law of magnetization M (x), as set forth by the conditions shown in Equation 10, to satisfy Equation 11. In the operation of demagnetization there is a tendency for the portion of the cylinders already demagnetized in a previous step to be further demagnetized'or the magnetic distribution otherwise altered by the demagnetization step for the succeeding portion of said cylinders. However, experience in performing this process will enable an operator to make allowances for this interaction between demagnetization steps and can thereby control the demagnetizing current to compensate for this interaction and produce a cylindrical magnet magnetized radially to provide a .substantially uniform axial magnetic field in accordance to the principles of this invention.

As an alternative method to 'magnetize the cylinders, as outlined above, two shells 30 and 31 having a length equal to one-half of the required magnet length may be fabricated from a plurality of individually magnetized radial segments, each segment being similar to segment 32. The segments forming shells 30 and 31 are radially magnetized by judicial winding of -a magnetizing coil such that the segments of shell 30 has -a magnetic polarity reversed with respect-to the magnetic polarity of the segments of shell 31as illustrated in Fig. 6. The segments of each ring are magnetized equally While those of adjacent rings are magnetized difieren't amounts according to the law of magnetization 'hereinbefore described.

While I have described above the principles of :my invention in connection with specific apparatus, it is to be form magnetic fieldis produced axially ojfsaid cylinder for themajor portion thereof.

.2. A permanentmagnet according to claim l wherein the radial magnetization is confined to individual segments arranged radially with respect to the axis of said cylinder with one polarity of each of said segments .on the inner surface of said cylinder and the opposite polarity at the outer surface of said cylinder.

3. A permanent magnet comprising a continuous, constant diameter, uniform wall hollow cylinder of magnetic material magnetized radially the entire ,length thereof, said radial magnetization having an opposite polarity on opposite sides of the center portion of said cylinder and .a gradient of magnetization from the ends of said cylinf8 der toward said center portion for production of a. sub stantially uniform magnetic field axially-of said cylinder as expressed by the relationship where a=iradius of s id cylinde MLz )'=leW o radial magnetization, x =distance along the longitudinal axis of said eylinder defining an arbitrary point therealong with reference to the central transverse axis thereof, and x=longitudinal distance from the said central axis to a ring element dx of said cylinder,

4. In a. travelingwave electron discharge device having meansfor producing agbeam of electrons for flow along a given path, a radio frequency propagating structure disposed adjacent said path, and means to apply radio frequency waves for fipw along said structure for interaction With electrons .of said "beam; a permanent magnet comprising a. continuous, constant diameter, uniform wall cylindrical shell of magnetic material disposed concentric of said path, said shell being radially magnetized for the entire length thereof, saidradial magnetization having opposite polarity on opposite sides of the center portion of said .shell and a gradient of magnetization from the ends of said shell toward said center portion to provide a magnetic ,field of substantially uniform strength axially of and substantially coextensive with said path.

'5. A device according to claim 4, wherein the gradient of radial magnetization on said shell produces a substantially uniform magnetic field as expressed by the re.- lationship where a=radius of said cylindrical shell, M(x)=law of radial magnetization, x =distance along the longitudinal axis of said shell defining an arbitrary point thenealong with reference to the central transverse axis thereof, and x=longitudinal distance from said central axis to a ring element dx of said shell.

References Cited in the file of this patent UNI IIED SIATES PATENTS (pages 276-277). 

